Deionized water (“DI-water”) and ultra pure water (used interchangeably herein) are commonly used in semiconductor device fabrication processes for rinsing or wet cleaning operations. However, use of a substantially non-conductive liquid such as DI-water in semiconductor fabrication processes can contribute to a buildup of charge on the surface of the wafer. This is especially a problem in fabrication processes utilizing spinning wafer tools, as electroosmotic effects produced by the contact between the wafer and the DI water used for cleaning operations can lead to charge buildup and eventual electrostatic discharge events. These discharge events can damage or even destroy structures on the wafer, or cause contaminants or undesirable particles to attach to the wafer.
Existing systems have sought to reduce charge buildup on the wafer during wet cleaning operations through the use of a conductive cleaning liquid. For example, a gas such as carbon dioxide (CO2) can be dissolved in the DI-water to produce carbonated deionized (“DI-CO2”) water.
Rinsing with conductive DI-CO2 water can avoid charge buildup on the wafer surface and allow for substantially damage-free cleaning while maintaining device integrity. CO2 has the further advantage of leaving substantially no solid residue as a result of evaporation, which is important in semiconductor processing. However, DI-CO2 water is acidic enough that it can undesirably etch away acid-sensitive materials such as copper and cobalt which are commonly used in the back end of line (“BEOL”) stage of wafer fabrication.
Another approach uses ammonia (“NH3”) instead of CO2. By dissolving NH3 in DI-water, an alkaline solution with substantially lower etch rates than DI-CO2 can be produced for use in wet cleaning operations.
NH3 can be supplied as a concentrated solution or as a gas. Due to the high solubility of NH3 in DI-water, use of NH3 in its gas phase results in a total absorption of the NH3 into the DI-water. However, NH3 gas is so reactive with DI-water that there is a high risk the DI-water will flow back into the NH3 gas supply line and into the NH3 valves when the flow rate of NH3 is sufficiently low. This can lead to serious control problems, as the flow characteristics of a valve are vastly different between a gas-filled valve and the same valve filled with water. It is therefore difficult to maintain a stable flow of NH3 gas into DI-water under such conditions, especially when the gas flow has to be interrupted from time to time in the normal course of the fabrication process.
Some systems have sought to avoid the challenges associated with precisely controlling the flow rate of NH3 gas by instead using a hollow-fiber membrane system to dissolve gas that is supplied at a substantially constant flow rate into DI-water having varying flow rates (e.g., between 1 L/min and 10 L/min). While these systems can deliver a liquid with stable conductivity under certain conditions, they do so by maintaining a 90% or higher saturation of the liquid, requiring that excess NH3 and other gases are supplied to the membrane system. This is not only an economic disadvantage, but also requires additional effort in the treatment of the undissolved off gas leaving the system and increases the risk in contamination of the ambient air with NH3.
For example, NH3 is a toxic gas and therefore special care is needed to avoid contamination of ambient air. The requirements for semiconductor fabrication are typically more restrictive regarding release of NH3 into the ambient air, as even NH3 concentrations well below typical environmental and health threshold limits can interfere with certain semiconductor manufacturing and processing steps.
Another approach is to avoid using NH3 gas altogether, and to instead dilute a concentrated NH3 solution into the DI-water. However, this approach requires a very high dilution rate that can be a factor 1000 and more. Further, accurately mixing such a small quantity of a liquid into another liquid is challenging due to the limited mixing time provided. Often, the limited mixing time results in fluctuations in the concentration of the NH3 in the liquid at the outlet to the system. This can be overcome by maintaining a constant flow rate between the NH3 solution and the DI-water. However, maintaining a constant flow rate is not preferable because of the high amount of liquid that is discharged at times when less liquid is needed for a particular operation. Utilizing a constant flow rate results in a large amount of wasted liquid discharge, and therefore substantially increases operating costs.
In addition, a concentrated aqueous NH3 solution absorbs CO2 from ambient air upon contact. CO2 can also permeate through the walls of the container or tank used to store the aqueous NH3 solution. This can lead to an increased carbonate content of the liquid over time, especially in systems that recycle a part of the supplied NH3 solution. The carbonate content can interfere with the process control, as the relation between the pH and the conductivity of an NH3 solution changes based on the carbonate content. Accordingly, systems based on the dissolution of aqueous NH3 require additional processing steps and components such as ion exchangers to remove the carbonate and other impurities from the supplied liquid.
Control of the concentration of the NH3 in the DI-water, or relatedly, control of the conductivity, at the dynamically changing DI-water flow rates that are typically required for single wafer applications is difficult. Typically, different conductivity set points are also requested and may take a long time to stabilize causing decreased throughput and therefore a higher cost of ownership. Further, due to a quadratic component between NH3 concentration and conductivity, a much wider range is needed for the NH3 flow than for the DI-water flow.
Precise steady state concentration for a constant flow can theoretically be achieved using a feedback control to eliminate all differences over time. However, accurate control of concentration is much more complicated for dynamically-changing flows of the cleaning liquid, as real systems cannot be built with a zero volume that would behave in an ideal manner. A real system has a certain volume that acts as a buffer volume during flow changes. Concentration changes are therefore often delayed, which leads to under dosage or concentration overshoot at changing flow rates, which influences conductivity. Such a behavior is unwanted and needs to be restricted to small variations in conductivity in order to maintain process stability at all conditions, and in order for each processing chamber to operate under the same conditions.